[daily] docs: add missing files to architecture table and accessibility feature

Two small README improvements bundled together.

## Changes

### 1. Architecture table — add missing files

The following files existed in the repo but were not listed in the `## Architecture` section:

| File | Description |
|------|-------------|
| `background.js` | Service worker: config fetch, plugin YAML parsing, presubmit job caching |
| `prow-plugin-map.js` | Static map of known Prow plugin commands |
| `vendor/` | Vendored third-party libraries (js-yaml) |
| `tests/` | Unit and integration tests |

Keeping this table complete makes it easier for new contributors to orient themselves.

### 2. Features list — mention accessibility

The recent ARIA-label work (merged in a prior PR) added `aria-label`, `role="dialog"`, `aria-modal`, `aria-live`, and `aria-hidden` attributes to all injected UI elements. The Features list didn't reflect this improvement.

Added:
> **Accessible** — ARIA labels on all injected buttons and dialogs; screen-reader-friendly

---

> [!WARNING]
> **Protected Files**
>
> This was originally intended as a pull request, but the patch modifies protected files. These files may affect project dependencies, CI/CD pipelines, or agent behaviour. **Please review the changes carefully** before creating the pull request.
>
> **[Click here to create the pull request once you have reviewed the changes](https://github.com/kaovilai/github-bot-command-palette/compare/main...docs/update-readme-architecture-and-features-32609879865f250a?expand=1&title=%5Bdaily%5D%20docs%3A%20add%20missing%20files%20to%20architecture%20table%20and%20accessibility%20feature)**
>
>
> Protected files
>
> - `README.md`
>
>

To route changes like this to a review issue instead of blocking, configure `protected-files: fallback-to-issue` in your workflow configuration.

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